For many years, battery-operated devices have been popular. In early devices, the user was required to power down the device or manually disconnect the battery from the device via a switch so that the device did not drain the battery when not in use.
There are now a number of electronic devices that have been developed to operate with low power so that completely isolating the batteries via a user-operated switch is not necessary. More recent electronic devices have also been designed with a sleep state wherein the microprocessor of the device will use a switch to cut power to nonessential elements of the device thereby saving additional energy by eliminating the leakage currents in those elements. When a user presses a button or otherwise attempts to use the device, the device wakes up and the processor causes power to be restored to the deactivated elements. Even in their sleep state, these low power devices still have leakage currents.
Even small leakage currents can significantly impact battery life. The following Table 1 illustrates one model of shelf life of a 9 volt battery and a comparable set of AA batteries.
TABLE 1SelfTotal CapacityDischarge(mAH)(2% per yr)9 V 580 mAH11.6 mAH/yrAA2700 mAH  54 mAH/yr
Table 1 assumes a self discharge rate of 2% per year of the capacity of a battery. Assuming now that a battery will last a year in a device and that the device is idle for 16 hours every day, then this self discharge rate is equivalent to an idle current drain of 9.2 microamps in the case of the AA batteries and 1.99 microamps in the case of the 9V battery.
Modem electronic devices are generally designed to use very little power while idle. This is generally accomplished by use of a sleep state in which most components are designed not to conduct any current. However, a CMOS device inherently allows some current flow called leakage current. Leakage currents present a significant impediment to battery life and energy conservation. The following Table 2 illustrates the amount of power dissipated by a device with different leakage currents assuming that the battery lasts at least a year in a device and that the device is in its sleep state for 16 hours every day.
TABLE 240 uA leakage5 uA leakage1 uA leakagePower% ofPower% ofPower% ofLosstotalLosstotalLosstotal(mAH/yr)capacity(mAH/yr)capacity(mAH/yr)capacity9 V233.640.28%29.25.03%5.841.01%AA233.6 8.65%29.21.08%5.840.22%
As shown in Table 2, leakage currents can present a significant energy drain during a sleep state and impact battery life. For the above-described usage scenario, almost half of the capacity (40.28%) of a 9V battery is dissipated by a device with only 40 microamps of leakage current in its sleep state. Reducing the leakage current to 1 microamp means that at the end of the year, the battery has retained over 39% of its capacity that it would have otherwise lost to leakage currents. Additionally, since only 1% of the battery's capacity is lost to leakage currents during the device's sleep state, the battery's own self discharge rate (approximately 2% per year) becomes a relatively important factor in the life of the battery. Thus, it is highly desirable to decrease the sleep state current draw of these devices to improve battery life and/or generally conserve energy. More specifically, it is desirable to substantially eliminate leakage currents within the device while it is in its sleep state, and still be able to easily and quickly wake the device from its sleep state.